For example, as for images of a metal, it often happens that the minimum distances between centroids of groups in various organizations differ from one another. Therefore it is important to calculate the minimum distance so as to find the boundary between different organizations.
It does not seem that an effective conventional method to calculate the minimum distance is published. In the case that the minimum distance is calculated by self-obvious method, it is necessary to calculate all of the centroid distances of each couple of groups, and moreover to find the minimum value among them. The number of distance calculations to find the minimum, it is: EQU N.times.N(N-1)/2N.sup.3
and as the number of groups increases, the calculation time grows.